1. Field
The invention relates to a force measuring cell with a deformable body and with at least one strain gauge installed on the deformable body, wherein the at least one strain gauge has a strain-sensitive electrical resistor track arranged on a carrier substrate and connector electrodes for contacting the resistor track, and wherein further the at least one strain gauge and in some cases at least a part of the deformable body are provided with a multilayer coating. The invention also relates to an individual strain gauge or to an array of strain gauges that are arranged in a row or over an area, wherein the one or more strain gauges have a strain-sensitive electrical resistor track arranged on a carrier substrate and a multilayer coating covering the resistor track and at least part of the carrier substrate. The invention further relates to a method of producing a multilayer coating on a strain gauge or on an array of strain gauges arranged in a row or over an area. In addition, the invention also relates to a force-measuring cell with a strain gauge.
2. Background Information
A strain gauge has a metallic resistor track arranged on a carrier substrate which can be made in the shape of a meandering structure by means of any known chemical etching method. Also arranged on the carrier substrate are connector electrodes for contacting the resistor track. The connector electrodes are often made in one work operation together with the resistor track, and they consist therefore in most cases of the same material. Electrically insulating materials are used for the carrier substrates of strain gauges. Depending on the area of application, one finds carrier substrates of glass, ceramic materials, in many cases polymers, glass-fiber reinforced polymers, or composite materials. Strain gauges are measuring elements in which a mechanical deformation causes a change of the electrical resistance and which are therefore used for the measurement of the force that produces the deformation.
In the field of weighing technology, to name an example, a force acting on a deformable body causes a deformation which is converted into an electrical signal by means of strain gauges. In a force-measuring cell that functions according to this principle, a load on the weighing pan which is connected to the vertically movable load-receiving part of the deformable body produces a displacement of the load-receiving part in relation to the spatially fixed part of the deformable body. In one embodiment, the deformable bodies used in force-measuring cells have four elastic bending zones formed by thin material portions which are located at the four corners of a parallelogram, so that the load-receiving part is arranged as a vertically movable leg of the parallelogram opposite a fixed, likewise vertical parallelogram leg that is preferably fastened to the housing of a weighing scale. The magnitude of the deformation that occurs in the thin bending zones is measured as an electrical resistance change by means of at least one strain gauge that is installed on one of the bending zones, in most cases by means of an electrically insulating adhesive layer.
Because of their elastic properties, polymer substrate materials are the preferred choice for strain gauges used in the field of weighing technology, in particular polyimides, but also epoxy resins, phenolic resins, melamines and ketones. Polymer carrier substrates have the advantage of a lower rigidity, so that their shape can conform more easily to the deformable body. This reduces in particular the mechanical stress in the adhesive layer. Hysteresis effects or a destruction of the adhesive layer that can occur when a rigid substrate is bonded to a deformable body are found far less often with polymer substrates. Furthermore, polymer substrates used for strain gauges with a meander-patterned resistor track offer the possibility of compensating a drift in the load signal through the known method of designing the return loops of the resistor track with an appropriately selected shape. Besides, strain gauges with polymer carrier substrates are easier to handle and more cost-effective to produce.
However, polymers have the disadvantage of a relatively high absorbance capacity for water and also for solvents, so that the humidity of the ambient air surrounding the load cell, and more particularly a change in the relative humidity, has a lasting influence on the measuring result. For example the sensitivity, the stability of the zero point and the creep properties, the so-called load drift, are parameters that are influenced by water- and solvent-related moisture in a force-measuring cell based on the strain gauge transducer principle. In measurements where the humidity of the ambient air surrounding a force-measuring cell was increased in one step from about 30% r.H. to 85% r.H. in the typical temperature range between 10° C. and 40° C., the change in this ambient parameter was found to cause a change in the weighing result of the order of some ten to a few hundred ppm (parts per million) of full span (full-load signal).
Some of the causes for the changes in the weighing results are understood and can be explained in physical terms. As a first factor, the substrate material of an unprotected strain gauge absorbs the moisture and therefore swells up, whereby the distance of the resistor track towards the bending zone is increased and the deformation that is imparted by the bending zone on the resistor track is changed by a small amount. As a second factor, the absorbed moisture changes the elastic properties of the substrate material and thereby changes the deformation parameters of the resistor track. As a third factor, an increased amount of moisture in the substrate material can cause leakage currents between neighboring parts of a meander-shaped resistor track or even between the resistor track and the metallic deformable body. While these effects are small in relation to the full-span signal, as shown by the aforementioned measurements, their influence on the measuring signal of a force-measuring cell that has to meet the highest accuracy requirements is nevertheless still unacceptably large. Protective devices and/or protective measures are therefore used in order to obtain a measuring signal that remains largely unaffected by conditions of the ambient environment, in particular by moisture acting on the substrate material and/or on the resistor track.
The known state of the art offers measures for the protection of strain gauges from moisture that causes a change in the measuring signal. For example DE 27 28 916 A1 describes the covering of a strain gauge installed on a measuring transducer. First, an electrically insulating layer is applied, for example a resin, or the strain gauge is imbedded in this layer so that a part of the transducer body that surrounds the strain gauge is likewise covered. A metallic layer is arranged on top of the electrically insulating layer and likewise covers a part of the transducer body. Thus, a strain gauge that is already installed on a transducer can be sealed against moisture effects.
A concept for protecting strain gauges against humidity is disclosed in U.S. Pat. No. 5,631,622, where an electrically insulating polymer coating is applied to the strain gauges and a metal foil is laminated onto the coating as an additional covering after a quantity of strain gauges have been produced in the form of an array on a sheet and before the sheet has been cut apart into the individual strain gauges. After the separating step, the metal foil still provides a large-area protective covering against humidity for each individual strain gauge.
As a means for protecting a strain gauge against corrosion and to improve the measuring properties, it is proposed in JP 7 113 697 A to stop moisture from entering by applying a thin inorganic film, for example SiO2 with a thickness of about 100 nanometers (nm), to the surface of the strain gauge as a kind of barrier against humidity. Subsequently, an inorganic insulating film, for example polyimide with a thickness of about 10 micrometers (μm) is applied which serves to plug microscopically small holes or breaks in the inorganic film, so-called pinholes through which moisture could still penetrate. The protection achieved by this dual-layer covering is not always satisfactory, in particular in highly sensitive force-measuring cells that are designed for relatively small loads.
A force transducer with strain gauges is disclosed in DE 40 15 666 C2, wherein a vapor-deposited diffusion-tight electrically insulating coating of silicon oxide or silicon carbide, preferably two to four micrometers thick, is applied to a strain gauge and the adjacent portion of the carrier substrate. Another embodiment can also have a coating of a silicon oxide layer at the bottom which is overlaid with a metallic layer, preferably a layer of nickel.
The solutions of the foregoing description suffer from the problem that the protective coatings or protective foils which form cover over the whole strain gauge, in particular the inorganic coatings or foils with a strong barrier effect, have a comparatively large mass and a high degree of stiffness so that they, too, cause a change in the measuring result produced by the strain gauge. This problem exists regardless of whether the protective coverings are applied directly to the strain gauge that is already installed on the measuring transducer or whether a covering is applied simultaneously to a large number of strain gauges following their production. The measurement errors are caused by so-called bypass forces that are caused by overlaying the strain gauge with a relatively thick coating or foil of the order of several microns as disclosed in the state-of-the-art references. Metal coverings or foils in particular, because of their comparatively high stiffness even if they are only a few microns (μm) thick, contribute measurably to a force bypass. A force bypass occurs for example as a result of thick inorganic protective coatings as they have a high stiffness of their own and thus contribute significantly to the overall stiffness of the aforementioned bending zones of the deformable body. This problem is particularly pronounced in force-measuring cells for the measurement of small forces, because the bending zones are in this case very thin in order to provide a high sensitivity. Consequently, undesirable changes of the elastic properties of the protective covering, such as for example an elastic after-effect (also known as creep), a high inelastic component, in particular a strain hysteresis, cause a measurement error that is not reproducible and therefore not amenable to software-based compensation techniques.
On the other hand, there is no question that passages for moisture that can occur particularly in very thin moisture barrier coverings, so-called pinholes as described in JP 7 113 697 A, need to be prevented or that at least their effects need to be reduced to the largest extent possible.
An adhesive strip which is described in EP 0 107 966 A2 has a polymer substrate topped by a layer of aluminum, and there is also an adhesive layer by means of which the adhesive strip is adhesively overlaid on a strain gauge that has been installed on a force-measuring cell, as a protective means against moisture penetration. In addition, the electrical connections of the strain gauge are covered by a caption foil.